While I was searching the web for some information related to depressed human immune response in space, I came across this tidbit about our sinuses.

Sinuses are cavities in the skull. As such, they help lighten the aforementioned bone mass. Some scientiets think that the sinuses help filter the air we breath in, regulating its temperature and helping remove impurities.

There are several sets of sinuses, all connected to the upper resperatory tract by passageways about the diameter of a pencil lead.

However, the drainage pattern of the sinues in humans is more suited to a quadraped than a biped, since some of our sinuses can drain by gravity only if we get down on all fours.

So while we evolved, some of our sinuses did not.

sp